Protein synthesis for secreted enzymes occurs on ribosomes bound to the rough ER

Steps:

• Enzymes are proteins composed of polypeptides synthesized by ribosomes.
• In cells secreting proteins, these ribosomes attach to the rough endoplasmic reticulum (RER) for co-translational translocation.
• Electron micrographs reveal RER as studded with ribosomes, indicating active protein synthesis sites.
• Thus, polypeptides for secreted enzymes originate at the RER.

Why C is correct:

• The rough ER hosts ribosomes that translate mRNA into polypeptides for secreted proteins, as defined by the endomembrane system's secretory pathway.

Why the others are wrong:

• A: The nucleus transcribes DNA to mRNA but does not synthesize polypeptides.
• B: Free ribosomes in cytosol produce non-secreted proteins, not those for enzyme secretion.
• D: The Golgi apparatus modifies and sorts proteins but does not initiate polypeptide synthesis.

Final answer: C